This invention relates to the field of emergency power sources which are automatically activated when ordinary utility power supplies fail, as during blackouts. More particuarly, the invention relates to the field of emergency power supplies of the type containing a storage battery which is charged during normal situations and discharged as an emergency supply during power outages.
All such systems have certain elements in common due to the fact that today's utility power is invariably alternating current (hereinafter AC) while there has been no such thing as an AC battery invented as yet. All batteries are direct current (DC). Therefore, to charge a battery from a utility line an inverter must be provided to convert the AC to DC; in order that the battery may supply the usual emergency equipment, which is commonly also AC, a second inverter must be supplied to convert the DC provided by the battery back to AC. This requirement is obviated, of course, if the load to be powered is itself DC; in such cases of course no inverter is required to convert the DC supplied by the battery to AC. If, as in a typical embodiment, the load to be powered under emergency situation is simply incandescent lighting, which may be DC, then no inverter is required. The present invention, however, is not of this type in that it is designed to supply AC loads under emergency conditions. Hence, an inverter is required in the class of devices to which the invention relates to convert the DC stored in the battery to AC.
A related area of prior art involves so-called UPS (uninterrupted power supply) systems. These systems are used as a sort of buffer between utility power and critical components such as modern digital computers, so that if the line voltage for any reason varies slightly or stops for a fraction of a second, power to the load is not lost; it being apparent that in many applications it is essential that power be continuously supplied to the device in question. For example, many modern computers use a type of memory to which power must be continually supplied, lest the information stored therein be lost. Hence, such UPS systems have been devised. The present application, however, is not concerned with such an uninterruptable power supply for reasons which will become clear hereinafter. Instead, the present application is directed towards a circuit and apparatus which is only useful as an emergency power supply. However, it is designed to be a far more efficient and practical emergency power supply than those found in the prior art and for this reason has a range of utility not covered by any prior art devices.